+# At this point, we need a real merge. No matter what strategy
+# we use, it would operate on the index, possibly affecting the
+# working tree, and when resolved cleanly, have the desired tree
+# in the index -- this means that the index must be in sync with
+# the $head commit.
+files=$(git-diff-index --cached --name-only $head) || exit
+if [ "$files" ]; then
+ echo >&2 "Dirty index: cannot merge (dirty: $files)"
+ exit 1
+fi